This invention relates to a fluidized bed combustion system and a method of operating same and, more particularly, to such a system and method in which a recycle heat exchanger is formed integrally with the furnace section of the system.
Fluidized bed combustion systems are well known and include a furnace section in which air is passed through a bed of particulate material, including a fossil fuel, such as coal, and a sorbent for the oxides of sulfur generated as a result of combustion of the coal, to fluidize the bed and to promote the combustion of the fuel at a relatively low temperature. These types of combustion systems are often used in steam generators in which water is passed in a heat exchange relationship to the fluidized bed to generate steam and permit high combustion efficiency and fuel flexibility, high sulfur adsorption and low nitrogen oxides emissions.
The most typical fluidized bed utilized in the furnace section of these type systems is commonly referred to as a "bubbling" fluidized bed in which the bed of particulate material has a relatively high density and a well-defined, or discrete, upper surface. Other types of systems utilize a "circulating" fluidized bed in which the fluidized bed density is below that of a typical bubbling fluidized bed, the fluidizing air velocity is equal to or greater than that of a bubbling bed, and the flue gases passing through the bed entrain a substantial amount of the fine particulate solids to the extent that they are substantially saturated therewith.
Circulating fluidized beds are characterized by relatively high internal and external solids recycling which makes them insensitive to fuel heat release patterns, thus minimizing temperature variations and, therefore, stabilizing the sulfur emissions at a low level. The high external solids recycling is achieved by disposing a cyclone separator at the furnace section outlet to receive the flue gases, and the solids entrained thereby, from the fluidized bed. The solids are separated from the flue gases in the separator and the flue gases are passed to a heat recovery area while the solids are recycled back to the furnace. This recycling improves the efficiency of the separator, and the resulting increase in the efficient use of sulfur adsorbent and fuel residence times reduces the adsorbent and fuel consumption.
In the operation of these types of fluidized beds, and, more particularly, those of the circulating type, there are several important considerations. For example, the flue gases and entrained solids must be maintained in the furnace section at a particular temperature (usually approximately 1600.degree. F.) consistent with proper sulfur capture by the adsorbent. As a result, the maximum heat capacity (head) of the flue gases passed to the heat recovery area and the maximum heat capacity of the separated solids recycled through the cyclone and to the furnace section are limited by this temperature. In a cycle requiring only superheat duty and no reheat duty, the heat content of the flue gases at the furnace section outlet is usually sufficient to provide the necessary heat for use in the heat recovery area of the steam generator downstream of the separator. Therefore, the heat content of the recycled solids is not needed.
However, in a steam generator using a circulating fluidized bed with sulfur capture and a cycle that requires reheat duty as well as superheater duty, the existing heat available in the flue gases at the furnace section outlet is not sufficient. At the same time, heat in the furnace cyclone recycle loop is in excess of the steam generator duty requirements. For such a cycle, the design must be such that the heat in the recycled solids must be utilized before the solids are reintroduced to the furnace section.
To provide this extra heat capacity, a recycle heat exchanger is sometimes located between the separator solids outlet and the fluidized bed of the furnace section. The recycle heat exchanger includes heat exchange surfaces and receives the separated solids from the separator and functions to transfer heat from the solids to the heat exchange surfaces at relatively high heat transfer rates before the solids are reintroduced to the furnace section. The heat from the heat exchange surfaces is then transferred to cooling circuits to supply reheat and/or superheat duty.
The simplest technique for controlling the amount of heat transfer in the recycle heat exchanger is to vary the level of solids therein. However, situations exist in which a sufficient degree of freedom in choosing the recycle bed height is not available, such as for example, when a minimum fluidized bed solids depth or pressure is required for reasons unrelated to heat transfer. In this case, the heat transfer may be controlled by utilizing "plug" valves or "L" valves for diverting a portion of the recycled solids so that they do not give up their heat in the recycle heat exchanger. The solids from the diverting path and from the heat exchanger path are then recombined, or each stream is directly routed to the furnace section, to complete the recycle path. In this manner, the proper transfer of heat to the heat exchanger surface is achieved for the unit load existing. However, these type arrangements require the use of moving parts within the solids system and/or need external solids flow conduits with associated aeration equipment which adds considerable cost to the system.
In order to reduce these costs, a system has been devised that is disclosed in U.S. application Ser. No. 371,170 filed on June 26, 1989 by the assignee of the present invention. According to this system, a recycle heat exchanger is provided for receiving the separated solids and distributing them back to the fluidized bed in the furnace section. The recycle heat exchanger is located externally of the furnace section of the system and includes an inlet chamber for receiving the solids discharged from the separators. Two additional chambers are provided which receive the solids from the inlet chamber. The solids are fluidized in the additional chambers and heat exchange surfaces are provided in one of the additional chambers for extracting heat from the solids. The solids in the additional chamber are permitted to flow into an outlet chamber when the level in the former chamber exceeds a predetermined height set by the height of an overflow weir. The solids entering the outlet chamber are then discharged back to the fluidized bed in the furnace section.
However, there are some disadvantages associated with this type of operation. For example, there is no dedicated structure for preventing the back-flow of separated solids from the furnace section to the outlet of the separator. Also, the space available for heat exchanger surfaces is limited, and pressure fluctuations in the furnace section are transmitted to the external heat exchanger which results in erratic performance. Also, the solids are directed from the heat exchanger through one discharge pipe to one relatively small area of the furnace section which is inconsistent with uniform mixing and distribution of the solids. Further, there is no provision for controlling the solids inventory, or furnace loading.